Methods of producing boards of lignocellulosic material are well known and have significant practical applications. The methods of manufacturing include the following main steps: breaking up the raw material to particles having a suitable size and/or fibers, drying to a predetermined moisture ratio and gluing of the material before or after the drying step, forming the glued material into a mat, which may be constructed of several layers, possibly also cold pre-compressing, pre-heating, water spraying of the surfaces, and heat compressing under pressure and heat in a stroke compressor or a continuous compressor until the board is finished.
During conventional heat compressing, the compressed material is heated primarily by using heated coils from adjacent heating plates or the steel bands. These have a temperature of between about 150 and 200.degree. Celsius depending on the type of product that is being compressed, the type of glue used, the desired capacity, etc. In this manner, the moisture in the material is evaporated closest to the heat sources so that a dry layer is developed in this area and the steam front gradually moves towards the center of the board from each side as the compression continues. When the dry layer has been developed this means that the temperature in this layer is at least 100.degree. Celsius, which initiates the curing of conventional glues. When the steam front has reached the center, the temperature at the center has reached at least 100.degree. Celsius and the boards even start to cure at their center, so that the compression can be stopped within a couple of seconds. This applies to situations when conventional urea formaldehyde glue (UF) and similar glues are used, such as melamine fortified (MUF) glue. When other glues, such as those having a higher curing temperature are used, then a higher temperature and a greater steam pressure is required in the board before any curing can begin.
To achieve the desired density, a compressor must apply a high surface pressure at a high temperature. This is not a problem for non-continuous compression in a so-called stroke compressor, but such compressors have other drawbacks, such as worse thickness tolerances, etc. When continuous compressors are used, the requirement for simultaneous high surface pressures and high temperatures have led to expensive high precision solutions with regard to the roller felt between the steel band and the heating plate positioned below. The method of providing heat to the board by means of heating coils makes the heating relatively time consuming, which results in long compression lengths (large compression surfaces).
The heating can also be achieved by delivering steam to the mat to be compressed. In this way, the heating time is drastically shortened and, in addition, the resistance of the material to compression is drastically reduced when steam is introduced so that less compression force and smaller compression surfaces are required. An injection box may be used to inject steam into the material mat, which, however, has certain drawbacks. To avoid these drawbacks, compression rollers have been developed that are perforated, and which function as a steam delivery member. Such an apparatus is disclosed in Swedish Patent No. 502,810.
Swedish Patent No. 502,272 describes a method that uses the advantages of steam heating in order to achieve the desired density profile of the finished board. The compression is performed in two steps wherein the mat in the first step is compressed to a moderate density having a substantially even density profile across the thickness thereof. In the second step, the mat is compressed to a higher density than the middle portion of the board. Between both of the steps the board is either fully cured or partially cured.
Although this represents an advance within the field of manufacturing particle boards, the method according to Swedish Patent No. 502,272 has the drawback that the two step compression with the intermediate step of curing makes the manufacturing process relatively complicated.
The use of steam injection for heating the material is well known in the industry. For example, European Patent No. 383,573; U.S. Pat. No. 2,480,851; British Patent No. 999,696; German Patent No. 2,640,686; German Patent Application No. 40 09 883; and Australian Patent No. 57390/86 show different examples of how steam is injected in continuous processes to produce fiber boards. Even these described processes require compression in two or more steps.
European Patent No. 383,572 further describes a process that only works with one compression step. In this process, the steam is introduced in a steam injection segment which apparently includes some type of conventional steam delivery through a steam box or similar device. When introducing steam in this manner, there is some sliding between the mat/weave and the steam box which creates substantial wear so that the sliding surfaces of the box must be replaced at regular intervals and problems with the sealing of the edges may occur as a result of the sliding of the fiber mat/weave relative to the steam box.
The object of the present invention is to provide a method of this kind that is simpler than the methods of the prior art and that avoids the drawbacks that are associated with conventional ways of introducing steam.